A register stack may comprise a number of general registers of a microprocessor, which have been designated for the storage of data required by, and pertaining to, procedures of a stored program being executed by the microprocessor. Specifically, upon execution of a particular procedure, a stack frame associated with that procedure is created within the register stack. The stack frame includes saved registers (containing variables local to relevant procedure), an activation record and a frame marker. When a procedure calls a further procedure, the called procedure in turn creates a further stacked frame on top of the stacked frame for the calling procedure. Accordingly, for a program in which a number of nested procedures are being executed, the register stack may include a corresponding number of stack frames.
A register stack accordingly allows multiple procedures to effectively share a large register file by stacking associated stack frames in on-chip registers. The call/return patterns of typical programs exhibit high call/return frequencies with small amplitudes. A register stack significantly reduces the number of stores (i.e. register saves) at procedures calls and loads (i.e. register restores) at procedure returns, and thus provide a "cache" of information pertinent to nested procedures. As processor operating frequencies are increasing faster than the access times to memory resources (e.g. RAM) are decreasing, reducing the number of memory accesses performed by a program, as a result of procedure calls and returns, provides a performance advantage. This advantage is becoming increasingly significant in view of the popularity of modular and object-oriented programming, and the use of dynamically linked libraries.
As a register stack has a limited size, it will be appreciated that in a deeply nested procedure call sequence, the capacity of the register stack may be exceeded in what is termed a "spill". In such situations, it is necessary to transfer some of the registers of previous stack frames from the register stack to a backing store in order to create capacity within the register stack. Similarly, as the register stack empties on procedure returns, it is desirable to restore registers of previous stack frames which have been stored in the backing store to the register stack. The transfer of stack frames between the register stack and the backing store may be problematic where the number of bits contained in a register of the register stack is not a multiple of the addressing scheme employed to address the memory resource where the backing store resides.